Welding 4130 Steel. ER70S-2 can be used but you will be undermatching the base material. Welding 4130 Chrome Moly Steel for Race Cars.
When a weldment is heat-treated it is put into a furnace at a predetermined temperature and cooling rate. Tips for welding 4130 and 4140 steel Many of the challenges and best practices for welding 4130 and 4140 steels are the same whether the application involves welding new material or making repairs. Preheat is generally not needed for thinner sections 0120-in but for tubing larger than 0120 a 300F-400F preheat is needed to obtain acceptable results.
You are specifing 4130 QT - however you will probably notice that you can get a wide range of properties with very similiar heat treatments.
The most commonly welded heat-treatable steels are 4130 4340 and 8630. 4140 steel is also commonly referred to as a chromoly steel or chrome moly steel. 4130 being an HTLA steel product form has high hardenability and is susceptible to hydrogen cracking in the weld metal and HAZ. The 4130 lacks the minimum required moly content 045 and exceeds the maximum carbon content 020 to be considered a chrome-moly steel product form.